Here's two things that might help you fix your problem. This is what fixed my issue. (Windows 10, POD HD 300 wasn't connecting due to driver issues of some sort). FYI I had my POD HD working previously but due to a power outage something got messed up and it stopped connecting.
1. In Device Manager go to the "Unrecognized Device" entry and click "Update Driver Software..". In the menus click "Browse my Computer for software" and then at the bottom "Let me pick from a list.." and then go to "Line 6" and see if your driver software is there. I tried installing mine but it got stuck in an infinite loop and i had to restart the computer.
2. Something I didn't realize at first is that the POD HD can install drivers even when it's unplugged from power. I plugged the POD HD USB cable in after doing step 1 while the thing was not powered and it still installed drivers. After that I plugged in power and it worked. For some reason having the thing powered on may affect the driver installation.
I hope these tips at least help you troubleshoot.